1. Field of the Invention
The present invention relates to an optical recording medium with multilayer structure, and a production method thereof.
2. Description of the Related Art
Up to now, optical recording media have spread widely because of their portability and ease of handling owing to non-contact recording and reproduction in an audio visual field, and have been applied recently as recording media which record a variety of information also in a computer field. In the audio visual field, an amount of information has increasing steadily because of digitization of images and audios, and enhancement of quality, and recording capacity requested for optical recording media in connection with it has also increased. In addition, the spread of small computers and diversification of information advance, and small-sized large-capacity optical recording media are requested. For this reason, also in optical recording media, a DVD (Digital Versatile Disk) having recording capacity of 4.7 GB was developed from a CD (compact disk) of 700 MB, and further, the recording capacity is enlarged to 8.5 GB by making a recording layer two layers.
Nevertheless, since not only television broadcasting is digitized from conventional analog broadcasting, but also it moves to High Definition Television (HDTV), image information broadcast has an amount of information which is four times the amount of the conventional analog broadcasting. Hence, further capacity increase of conventional optical recording media is desired
As means of increasing the capacity of an optical recording medium, roughly speaking, there are two methods. That is, they are a method of enhancing a recording density by making the pitches of recording tracks narrower to make recording marks smaller, and a method of enhancing an apparent recording density by forming the recording layer into a multilayer.
The method of enhancing a recording density by making the pitches of recording tracks narrower to make recording marks smaller is achieved by shortening a wavelength λ of an irradiated laser and enlarging a numerical aperture (NA) of an objective lens.
Japanese Patent Application Laid-Open No. 2001-357571 proposes an optical recording medium using an optical system developed from λ=0.65 μm and NA=0.60 which is an optical system of a digital versatile disk (DVD). Now, a blu-ray disc using an optical system of λ=0.405 μm and NA=0.85, and a high definition DVD (HD DVD) using an optical system of λ=0.405 μm and NA=0.65 are studied and developed, which is planed to be marketed in 2006.
Furthermore, in order to increase a recording capacity, there is a method of forming a recording layer into a multilayer.
Since the HD DVD has a recording layer provided in a center portion of a 1.2 mm-thick substrate similarly to the DVD, the manufacturing techniques of the DVD are applicable. On the other hand, since a recording layer is located in a deep position from a substrate, it is difficult to enlarge a numerical aperture. Hence, a beam spot diameter cannot be decreased to a wavelength limit. Moreover, since a recording layer is located in a deep position from a substrate, it is difficult to form a recording layer into a multilayer.
On the other hand, in the Blu-ray Disc (hereinafter, abbreviated as “BD”), a beam spot can be made small upto a wavelength limit by enlarging a numerical aperture (NA) of an objective lens. But, an optical recording medium becomes remarkably weak also with respect to an inclination. For this reason, since it is necessary to arrange a recording layer near the surface of a substrate, the recording layer is formed on the surface of an optical recording medium, and the disc has structure of providing an organic protective layer having a thickness of 0.1 mm on the recording layer. Furthermore, since a recording layer is located on the surface of a substrate, formation of a multilayer is relatively easy, and hence, although being a prototype in an early stage of development, an example of an eight-layered recording layer is announced. In addition, also in the BD, the thickness of a substrate is 1.2 mm.
Record pits as data, or a land, a groove, record pits for addressing, and the like which were beforehand formed in the substrate are hereinafter collectively called a signal pattern. In addition, layers, such as a reflective layer or a stacked layer composed of a reflective layer and a recording layer which record data, stacked on the surface on which the signal pattern is formed, are collectively called a recording layer.
Japanese Patent Application Laid-Open Nos. 2002-260307, 2004-30885, 2004-220750 and the like disclose production methods of BD having two-layer structure. The production method of a BD which is disclosed in Japanese Patent Application Laid-Open No. 2002-260307 will be described in detail with referring to FIGS. 9A, 9B, 9C, 9D, 9E, 9F, 9G and 9H. Since a photopolymer (hereafter, abbreviated as “2P resin”) is used for a resin of the second layer and transfer to the 2P resin is performed, this production method is hereinafter called “2P method”. The 2P method will be described below in detail with referring to schematic sectional views of steps shown by FIGS. 9A, 9B, 9C, 9D, 9E, 9F, 9G and 9H.
As the 2P resin used, there are acrylic resins, cationic resins, thermosetting resins and the like, and it is possible to use an optimum material at a step to need the material.
(1) A signal pattern formed in a metal stamper 14 is transferred to a substrate 1 made from a polycarbonate resin (hereinafter, abbreviated as “PC resin”) by using an injection molding method (FIG. 9A).
(2) An aimed recording layer 4 is formed on the above-mentioned signal pattern (FIG. 9B).
(3) A transparent stamper 2 for forming a signal pattern which becomes the second layer is injection molded at the same time by using the stamper 14 (FIG. 9C).
(4) Then, the signal pattern is transferred to the 2P resin 3 by coating the 2P resin 3 on the recording layer 4, pressing the transparent stamper 2 on the 2P resin 3 and curing the 2P resin 3 (FIG. 9D).
(5) An interlayer 25 is formed by peeling off the transparent stamper 2 (FIG. 9E). The transparent stamper 2 is discarded (FIG. 9H).
(6) Further, an aimed recording layer 5 is formed on the interlayer 25 (FIG. 9F).
(7) Then, an organic protective film 8 is formed thereon to complete a two-layer optical recording medium (FIG. 9G).
In the case of producing a multilayer optical recording medium with three or more layers, the multilayer optical recording medium with three or more layers can be obtained by repeating the steps (3) to (6) after forming the recording film 5.
In Japanese Patent Application Laid-Open No. 2002-260307, a PC resin is also used for the transparent stamper 2 as well as the substrate 1. But, in order to improve the peeling property from the 2P resin, a metal film having a thickness of 100 nm and containing Al or Si as a main component is formed using a sputtering method.
As to a shape of the substrate, as illustrated in FIG. 10, convex-concave pits 113 for becoming a signal pattern are formed on the surface of a first substrate 111, an Al reflective film 115 having a thickness of 100 nm is formed so as to cover the convex-concave pits 113, and an information recording layer SA 116 is composed of the convex-concave pits 113 and Al reflective film 115.
In the above-described 2P method, since a degree of adhesion between the 2P resin and PC resin was good, in order to peel the transparent stamper from the 2P resin, it was necessary to use poly methylmethacrylate (PMMA), or cyclo olefin polymer (COP) having poor adhesion property with the 2P resin, that is, good peeling property from the 2P resin. Alternatively, after forming the transparent stamper using the PC resin, it was necessary to form a metal film on the formed surface of the transfer pattern (signal pattern) of the transparent stamper by using a sputtering method.
When using an olefin resin or the like having poor adhesion property with the 2P resin for the transparent stamper, the following problems arise.
1. Since it is necessary to perform injection molding using two kinds of resins, that is, the PC resin used for the substrate, and the olefin resin or the like used for the transparent stamper, an injection molding machine for forming the transparent stamper and the substrate is required.
2. It is necessary to manage two kinds of resins of the PC resin and the olefin resin or the like.
When forming a metal film on the surface of the transparent stamper made of the PC resin, the following problems might arise.
1. A sputtering apparatus for forming the metal film was necessary in addition to a sputtering apparatus for forming the recording layer, and
2. It took time for ultraviolet curing since transmittance of ultraviolet rays is reduced by the metal film.
Furthermore, there was a problem that the transparent stamper made of a resin could not used repeatedly because of stress at the time of peeling. There was also the same problem as at the time of using the olefin resin having poor adhesion property with the P resin, in the case of forming the metal film.
On the other hand, although the adhesion property between the 2P resin and the substrate is good, the adhesion property between the film for forming the recording layer and the 2P resin, or the adhesion property between the recording layer and the 2P resin is poorer than the adhesive between the 2P resin and the substrate. Hence, even when using the olefin resin or the like which is easily peelable from the 2P resin for the transparent stamper, peeling between the 2P resin and the recording layer, or between the substrate and the recording layer may occur. Therefore, further improvement is requested.